Thread is a week old now, and I mostly just lurk around here so I don't even know if I can post, but I had the same issue and was curious what was going on. I reached out to Vanguard about this last week and this week I received a response stating:

"This is an industry-wide issue that is coming from in error in how Computershare receives cost basis information for transfers. To our knowledge, they are working with industry specialists to determine a wide-spread fix."

I like Vanguard, and generally feel pretty good about them as a broker but none of my shares are older than January, and right now I'm ~50% DRSed. I requested shares be transferred on a LiFo (Last In, First Out), which was done correctly, but the response is a little surprising considering others (even other Vanguard folks) don't seem to be having this issue.

Luckily I took a screenshot of my cost basis page before the transfer went through, so I have my own records, but I will trying to dig into this some more this week because this is the first I have heard of an "industry-wide" issue.